The present invention relates to a waveform generator for an analog electric signal.
The aim of such a device is to produce an analog electric signal having a well determined shape from digital information applied in the form of digital word sequences, each word representing an increment of the analog signal.
In an exemplary embodiment, each of the words may comprise three bits, so that there are at the most six possible increment values for the analog signal. The generator periodically receives a 3-bit word, and the output signal is incremented by a value which depends upon the word value.
Before describing the general structure of the generator, an exemplary embodiment will be disclosed to show the requirements that are to be met for realizing the generator.
The chosen example relates to a telecommunication circuit comprising a telephone line which has to emit sinusoid portions having a determined frequency.
The sinusoid portions are determined in the form of a series of digital words representing the positive or negative increments that are to be constantly added to the signal so that this signal actually represents a sinusoid portion.
Digital words comprising only one bit representing an increment sign could be envisaged: +V or -V increment; but then the maximum output slope of the analog signal is limited since form one sampling period to the next one it is not possible to increase the analog signal by more than one increment. Indeed, this increment necessarily has a relatively low value with respect to the overall amplitude of the analog signal, if it is desired to realize low slopes for the output signal without being compelled to provide for the possibility of generating null increments (which is not desirable for transmission mode reasons).
That is the reason why it is chosen to define the analog signal by increments having several possible amplitudes; in the example chosen for illustrating the problem, the digital words have 3 bits permitting to determine 5 possible values, which are: 1 null central value, 2 positive values and 2 negative values, the absolute values of which are equal to the positive values.
However, still in the same example, it has been provided that the positive values are in a ratio equal to 3.
In other words, if V is the increment value of the elementary analog signal, the 5 possible increment values are: EQU -3V, -V , O , V , 3V
It will be noted later on that this ratio 3 plays an essential role (however, the ratio could be different, for example equal to 4 or 5).
The digital words can be applied to a very high input frequency, the output analog signal exhibiting the form of a step signal wherein the sampling period is substantially higher than twice the maximum frequency of the analog signal spectrum to be produced; in this way, there is no problem in smoothing the signal through a non-sampled low-pass filter, positioned at the generator output.
Typically, the sampling frequency can be 15 MHz for an analog signal, the spectrum of which is limited to 80 KHz.
The principle of restoring an analog signal by a series of digital words representing a very low number of possible increments is therefore very advantageous since it limits the complexity of the digital/analog conversion circuits (it is easier to process 3 bits at high frequency that 12 bits) and it facilitates the smoothing filter realization at the output (this filter is different to form when the sampling frequency is too close to twice the maximum frequency of the analog signal to be produced).
However, in order for the waveform generator to operate satisfactorily, it is then necessary that the value of the increments be accurately determined, failing which the signal produced is not the signal that is expected. This is easily understood since an integration of successive increments is carried out: a small difference in one increment may, if it is often repeated, lead to a very important trouble in the shape of the produced signal.
One of the objects of the invention is to provide for an analog signal waveform generator that supplies signal increments as accurate as possible and, in particular, increments that are in close relative ratio the one with respect to the other when several increment values are liable to be generated as a function of the input digital word.